1. Field of the Invention
The present invention relates to a method for producing a photomask and, more particularly, it relates to a method for easily producing a photomask having high accuracy and high quality using a silver halide photographic material.
2. Description of the Prior Art
It is known in the field of printed circuit fabrication or microelectronic fabrication to use a photomask in forming images utilizing a photoresist method. Heretofore, the photomask has often utilized silver images or chromium images. Silver images are obtained by imagewise exposing a photographic light-sensitive material (emulsion mask) provided on a glass plate, followed by ordinary photographic processings (e.g., development and fixing). On the other hand, chromium images are formed by providing a vacuum-deposited chromium layer on a glass support and etching the chromium layer using a photo etching method. That is, a photo resist is coated on the chromium layer and, after exposure through a photo mask superposed thereon (contact method) or after projecting a photo mask on to the photoresist (projection method), the photoresist is development-processed to form a resist image, followed by etching the chromium layer.
Since an emulsion mask has high sensitivity, it is suitable for preparing a photo mask. However, the resolving power of the photo mask obtained is inferior to that of the chromium mask using the projection method due to scattering of light by the silver grains, high thickness of the silver halide emulsion layer and the silver particles after development. A photo mask utilizing a silver image is inferior in resolving power for the following additional reason. That is, since the developed emulsion thickness is larger at silver image areas that at non-image areas, gaps or spaces are formed at the contact surfaces upon intimately contacting the photo mask with the photoresist layer in order to form a resist image, thus markedly reducing the resolving power. In addition, the photo mask formed by the silver image has such as small mechanical strength that it is easily damaged. Also, since a silver image does not transmit visible light, positioning of the mask is difficult.
On the other hand, a chromium mask using a contact method is inferior to the photo mask obtained from a silver image in resolving power, since the former is reduced in resolving power upon exposing a photoresist layer and upon etching the vacuum-deposited chromium layer. Also, with a chromium mask using the projection method, the photoresist layer has such a low sensitivity that exposure requires too much time, and thus it is not practical industrially.